Transistors are widely used components in devices and articles such as, for example, liquid crystal display devices ("LCDs"), Notebook Personal Computers (PCs), calculators, watches, liquid crystal color TVs, toys, word processors, and the like. There are several kinds of transistors such as, for example, Field Effect Transistors ("FETs"), Thin Film Transistors ("TFTs"), and the like. However, they all generally have three main elements in them: However, they all generally have three main elements in them: a substrate, a semiconductor (or semiconducting) layer and electrodes attached to the semiconducting layer.
In a common construction of a TFT/FET, the substrate, which is typically a single crystalline silicon wafer is coated with an insulating layer, typically silicon dioxide grown directly on the silicon. Generally two electrodes (the source and drain electrodes) are attached to the insulating layer. The silicon crystal acts as the gate electrode by being appropriately attached via a conducting paste to the silicon. The semiconductor layer is deposited on the insulator. Generally the semiconducting material is inorganic.
While transistors built on silicon and similar substrates have enjoyed much commercial success, there has been a perceptible move toward other substrates recently. Typically the silicon-based transistors are rigid and not flexible since they are built on rigid substrates. Therefore, where a flexible device is desired, the silicon based transistors have less utility. There are problems associated with breaking, delamination and the like. A typical example is a smart card. Smart cards are currently manufactured with silicon-based semiconductors and require substantial protection from abuse, bent, damaging of functionalities on the card and the like, thus adding significant cost to manufacture them. A flexible substrate like an organic substrate will be of substantial benefit to such devices. This is true of many toys, miniature or space saving electronic gadgets and the like.
Furthermore, orientation of the substrate is important when using thin film semiconducting materials. This is particularly true when using organic semiconducting materials (see below). The orientation of the semiconductor lo layer depends on the orientation of the substrate. In order for the semiconductor to become oriented, the substrate has to be a nearly defect free single crystal since even slight defects affect the orientation of the semiconductor layer. Otherwise expensive and generally low yielding thermal annealing process has to be used.
Attempts have been made to use organic semiconductor materials on silicon. H. Akimichi et al, Applied Physics Letters, Vol. 58 (14), 1500 (1991) describe FETs containing alkyl substituted oligothiophenes as the semiconducting material. A. Dodabalapur et al, Science, Vol. 268, 270 (1995) describe organic transistors containing the oligomer, .alpha.-hexathienylene (".alpha.-6T"), as the semiconductor. F. Garnier et al, Science, Vol. 265, 1684 (1994) describe FETs containing .alpha.,.omega.-di(hexyl)sexithiophene as the semiconductor material.
However, there are other problems that occur when one uses an organic semiconductor on an inorganic substrate. The bonding between the two layers is less than desirable limiting the utility of the device. Without strong chemical interaction between the substrate and the semiconducting material, it is very difficult to deposit crystalline thin films. Organic substrates are therefore preferred for organic semiconducting materials. In the case of inorganic semiconducting materials, such as silicon-based semiconductors, generally annealing procedures are used, especially for manufacturing liquid crystal display devices ("LCDs") (such as, for example, the active matrix devices and super twisted nematic devices) because of relatively high lattice energies of such substrate materials. However, the annealing processes require over 1000.degree. C. temperatures. The use of suitable organic substrates may permit the omission of such annealing step. Furthermore, In general, due to the low lattice energies associated with many organic semiconducting materials, annealing processes do not work properly, thus resulting in low manufacturing yields. It will, therefore, be useful if suitable organic substrates are available that are sufficiently transparent, can be oriented and processed at low temperatures and thus be useful for transistor fabrication.
It is, therefore, an object of this invention to provide transistors containing organic polymeric substrates that are transparent and can be oriented.
It is an additional object of this invention to provide devices containing transistors which contain organic polymeric substrates.
It is a further object of this invention to provide transistors containing organic polymeric substrates and organic semiconductors.
It is yet another object of this invention to provide oriented organic polymeric substrates with substantially reduced defects for transistor applications.
Other objects and advantages of the present invention shall become apparent from the accompanying summary, drawings and description.